It is well known that coking is a severe thermal cracking process in which one of the end products comprises carbon, i.e. coke. The delayed coking process was initially developed to minimize refinery yields of residual fuel oil by severe cracking of feedstocks such as vacuum residuals and thermal tars to produce coke and lower molecular weight hydrocarbons. U.S. Pat. Nos. 4,049,538 and 4,547,284, the disclosures of which are incorporated herein by reference, show examples of delayed coking processes.
It is also well recognized that the delayed coking process generally involves heating the feedstock in the conduit or tubing of a tube heater to a temperature above the cracking temperature while feeding the feedstock at a high velocity through the conduit. The optimum operation involves the use of feed rate such as to minimize the actual formation of carbon in the heated conduit of the tube heater. The tube heaters are often referred to interchangeably as coker heaters or coker preheaters and the terms are similarly used interchangeably in this description.
In U.S. Pat. No. 4,049,538 a coker preheater is illustrated diagrammatically as item number 11. In U.S. Pat. No. 4,547,284 a coker heater is illustrated diagrammatically as item number 25. The heated feedstock at the coking temperature is passed from the heating zone to a coke drum wherein preferably the majority of the coke formation takes place. In the insulated coke drum, or surge drum, a sufficient residence time allows the coking to take place. Typically, the heated coking feedstock has been heated to a temperature sufficient to maintain the coking in the drum, i.e. temperature in the range of about 750 to about 975° F. (399 to 524° C.). As the process proceeds, coke accumulates in the coking drum and is later removed by techniques known in the art.
Although much effort has been devoted in the past to providing conditions that will allow for the delayed coking feedstock to be heated to the cracking temperature without the formation of undesirable carbon deposits in the conduits or tubes of the coker heater, carbon deposition in the conduits of the coker heater still continues to be a problem.
As coke deposits in the conduit of the tube heater, the flow of feedstock through the heater is restricted. The restriction of flow can lead to increased residence time that in turn can lead to the deposition of additional coke. The coke deposits in turn tend to insulate the tube so that more heat must be applied to achieve the same rate of heating of the feedstock. In addition, the coke deposits cause the tubes to become much hotter. All these factors obviously tend to encourage the formation of still more coke within the tube of the tube heater further exacerbating the problem.
If the temperature of the tube increases enough, a tube rupture can occur. The likelihood of tube rupture is also aggravated by the fact that the feed must be pumped at ever-higher pressures as the flow is restricted by coke deposition in the tubes of the heater. The combination of exposing the tubes to higher temperatures and higher pressures greatly increases the probability of tube rupture and total shut down of the delayed coking process.
Because of the formation of coke deposits in the tubes of the heaters, operators of coke furnaces in the past have had to periodically shut down the operation and remove the coke that had been formed within the tubes of the heater.
It would be desirable if a cracking heater such as a coke furnace could be devised to minimize coke deposition within the heater tubes and increase the efficiency with which the feedstock in those tubes is heated. If such a furnace could be devised which additionally has reduced volume, this additional characteristic would be advantageous.